One of the techniques for achieving large capacity optical recording media is three-dimensional recording/reproducing. Three-dimensional recording/reproducing utilizes the dimension in the thickness direction in addition to the dimension in the in-plane direction of an optical recording medium, thereby three-dimensionally recording/reproducing information on an optical recording medium.
One of the three-dimensional recording/reproducing techniques is called microhologram recording. In microhologram recording, two facing lights are made to converge at the same point in the recording layer of an optical recording medium so that they interfere with each other and form a small diffraction grating near the convergence point for recording information. Then, either one of the two lights is made to converge at the diffraction grating and the reflected light from the diffraction grating is received for reproducing the information.
Non-Patent Literature 1 describes an optical unit used in the above microhologram recording. FIG. 19 shows the optical unit described in the Non-Patent Literature 1. Light emerging from a laser 143 has the beam diameter enlarged after transmission through a beam expander 144 and, after transmission through a λ/2 plate 145, becomes a linearly polarized light having a polarization direction of 45° with respect to the sheet surface in a plane perpendicular to the optical axis. Approximately 50% of the light is transmitted through a polarized beam splitter 146 and approximately 50% is reflected by the polarized beam splitter 146.
For recording information on a recording medium 158, the light transmitted through the polarized beam splitter 146 is reflected by mirrors 153 and 154 and almost 100% of the light is transmitted through a polarized beam splitter 155. Transmitted through a λ/4 plate 156, the light is transformed from a linearly polarized light to a circularly polarized light. An objective lens 157 converges the light in the recording layer of a recording medium 158. On the other hand, the light reflected by the polarized beam splitter 146 is reflected by mirrors 147, 148, and 149, passes through a shutter 150, and is transmitted through a λ/4 plate 151, whereby it is transformed from a linearly polarized light to a circularly polarized light rotating in the opposite direction to the above circularly polarized light. An objective lens 152 converges the light in the recording layer of the recording medium 158.
For reproducing information from the recording medium 158, the light transmitted through the polarized beam splitter 146 is heading for the recording medium 158 in the same way as for recording. Meanwhile, with the shutter 150 being closed, the light reflected by the polarized beam splitter 146 is blocked by the shutter 150, not heading for the recording medium 158. The light transmitted through the polarized beam splitter 146 converges in the recording layer of the recording medium 158. The light is partly reflected by the recording layer of the recording medium 158, passes through the objective lens 157 in the reverse direction, and is transmitted through the λ/4 plate 156, whereby it is transformed from a circularly polarized light to a linearly polarized light having a polarization direction perpendicular to that in the outward travel. Almost 100% of the light is reflected by the polarized beam splitter 155 and converged by a convex lens 159 on the light reception part of a detector 160.
Another three-dimensional recording/reproducing technique is called page hologram recording. For recording in page hologram recording, two lights called information light and reference light are used. The intensity profile of information light in a cross-section perpendicular to the optical axis is modulated according to recording data. Then, the two lights are made to enter the recording layer of an optical recording medium so as to form a hologram in the recording layer for recording information. For reproduction, only the reference light of the two lights is made to enter the recording layer of the recording medium to detect the intensity profile of the diffracted light from the hologram in a cross-section perpendicular to the optical axis for reproducing information.
Patent Literature 1 describes a recording medium and optical unit used in the above page hologram recording. The recording medium described in the Patent Literature 1 utilizes azobenzene as the material of the recording layer. Azobenzene exhibits birefringence, having a higher refractive index to the polarization components parallel to the molecular orientation than to the polarization components perpendicular to the molecular orientation. Irradiated with a linearly polarized light, the molecules are oriented in the direction perpendicular to the polarization direction. Then, the polarization state of light can be recorded. In the optical unit described in the Patent Literature 1, the information light and reference light enter the recording layer of a recording medium as P polarization and S polarization, respectively. The polarization state of the composite light of the two is recorded in the recording layer as hologram. Here, when only the reference light enters the recording layer of the recording medium as S polarization, the diffracted light from the hologram exhibits P polarization.    Patent Literature 1: Unexamined Japanese Patent Application KOKAI Publication No. 2005-316279.    Non-Patent Literature 1: Japanese Journal of Applied Physics, Vol. 45, No. 2B, pp. 1239-1245 (2006).